SRM/MRM assay for the mesothelin (MSLN) protein

ABSTRACT

Methods are provided for detecting and quantifying the Mesothelin protein (MSLN) in biological samples that have been fixed in formalin using Selected Reaction Monitoring (SRM)/Multiple Reaction Monitoring (MRM) mass spectrometry. The biological sample may be, for example, tissues and cells treated with formaldehyde containing agents/fixatives including formalin-fixed tissue/cells, formalin-fixed/paraffin embedded (FFPE) tissue/cells, FFPE tissue blocks and cells from those blocks, and tissue culture cells that have been formalin fixed and or paraffin embedded. A protein sample is prepared from the biological sample and the MSLN protein is quantitated by SRM/MRM mass spectrometry by quantitating one or more MSLN fragment peptides in the protein sample.

This application claims priority to provisional application Ser. No. 62/161,506, filed May 14, 2015, the contents of which are hereby incorporated by reference in their entirety. This application also contains a sequence listing submitted electronically via EFS-web, which serves as both the paper copy and the computer readable form (CRF) and consists of a file entitled “001152_8043_US01_SEQ_LISTING”, which was created on May 13, 2016, which is 607 bytes in size, and which is also incorporated by reference in its entirety.

INTRODUCTION

Cancer is treated with a collection of therapeutic agents that kill growing and dividing cells and that function in a variety of ways. A common collection of chemotherapeutic agents has been used for decades, either individually or in combinations, and this common collection of agents has become the traditional and routine cancer treatment in clinical oncology practice. These traditional chemotherapeutics agents act by killing all cells that divide rapidly—rapid cell division being one of the main properties of most cancer cells. However, these agents also kill growing normal cells and, accordingly, these agents are not considered to be “targeted” approaches to killing cancer cells. In recent years a large group of cancer therapeutic agents has been developed that selectively target cancer cells, where the therapeutic agent specifically attacks a protein that either is expressed only by the cancer cells and not by normal cells, or that is expressed in much higher amounts in cancer cells than in normal cells. This approach is considered to be a “targeted” approach to cancer therapy. Most recently, another approach to killing cancer cells in a “targeted” fashion has been to specifically modulate the immune system to enhance the ability of the cancer patient's immune system to kill cancer cells.

Therapeutic agents that target the Mesothelin protein, which is also referred to as MSLN, have shown promise in early clinical trials. However, only those patients whose cancer cells express high amounts of the MSLN protein would be most likely to benefit from treatment with these MSLN-targeted therapeutic agents. The methods described below provide a quantitative proteomics-based assay that delivers a relevant measure of activation of the MSLN signal pathway as MSLN is not normally expressed in normal tissue and/or normal epithelial cells. In particular, the methods provide a mass spectrometry assay that quantifies MSLN in formalin fixed tissues from cancer patients and that enables improved treatment decisions for cancer therapy.

Specific peptides derived from subsequences of the Mesothelin protein, (also referred to as CAK1, pre-pro-megakaryocyte-potentiating factor, megakaryocyte-potentiating factor, and MPF, and which is referred to herein as MSLN, are provided. The peptide sequence and fragmentation/transition ions for each peptide are particularly useful in a mass spectrometry-based Selected Reaction Monitoring (SRM), which can also be referred to as a Multiple Reaction Monitoring (MRM) assay, referred to herein as SRM/MRM. The use of peptides for SRM/MRM quantitative analysis of the MSLN protein is described.

This SRM/MRM assay can be used to measure relative or absolute quantitative levels of one or more of the specific peptides from the MSLN protein and therefore provide mass spectrometric methods of measuring the amount of the MSLN protein in a given protein preparation obtained from a biological sample.

More specifically, the SRM/MRM assay can measure these peptides directly in complex protein lysate samples prepared from cells procured from patient tissue samples, such as formalin fixed cancer patient tissue. Methods of preparing protein samples from formalin-fixed tissue are described in U.S. Pat. No. 7,473,532, the contents of which are hereby incorporated by references in their entirety. The methods described in U.S. Pat. No. 7,473,532 may conveniently be carried out using Liquid Tissue® reagents and protocol available from Expression Pathology Inc. (Rockville, Md.).

The most widely and advantageously available form of tissues from cancer patients tissue is formalin fixed, paraffin embedded tissue. Formaldehyde/formalin fixation of surgically removed tissue is by far the most common method of preserving cancer tissue samples worldwide and is the accepted convention for standard pathology practice. Aqueous solutions of formaldehyde are referred to as formalin. “100%” formalin consists of a saturated solution of formaldehyde (about 40% by volume or 37% by mass) in water, with a small amount of stabilizer, usually methanol to limit oxidation and degree of polymerization. The most common way in which tissue is preserved is to soak whole tissue for extended periods of time (8 hours to 48 hours) in aqueous formaldehyde, commonly termed 10% neutral buffered formalin, followed by embedding the fixed whole tissue in paraffin wax for long term storage at room temperature. Thus molecular analytical methods to analyze formalin fixed cancer tissue will be the most accepted and heavily utilized methods for analysis of cancer patient tissue.

Results from the SRM/MRM assay can be used to correlate accurate and precise quantitative levels of the MSLN protein within the specific tissue samples (e.g., cancer tissue sample) of the patient or subject from whom the tissue (biological sample) was collected and preserved. This not only provides diagnostic and prognostic information about the cancer, but also permits a physician or other medical professional to more accurately determine appropriate therapy for the patient. Such an assay that provides diagnostically, prognostically, and therapeutically important information about levels of protein expression in a diseased tissue or other patient sample is termed a companion diagnostic assay. For example, such an assay can be designed to diagnose the stage or degree of a cancer and determine a therapeutic agent to which a patient is most likely to respond.

SUMMARY

The assays described herein measure relative or absolute levels of specific unmodified peptides from the MSLN protein and also can measure absolute or relative levels of specific modified peptides from the MSLN protein. Examples of modifications include phosphorylated amino acid residues and glycosylated amino acid residues that may be present on the peptides.

Relative quantitative levels of the MSLN protein are determined by the SRM/MRM methodology, for example by comparing SRM/MRM signature peak areas (e.g., signature peak area or integrated fragment ion intensity) of an individual MSLN peptide in different samples. Alternatively, it is possible to compare multiple SRM/MRM signature peak areas for multiple MSLN signature peptides, where each peptide has its own specific SRM/MRM signature peak, to determine the relative MSLN protein content in one biological sample with the MSLN protein content in one or more additional or different biological samples. In this way, the amount of a particular peptide, or peptides, from the MSLN protein, and therefore the amount of the MSLN protein, is determined relative to the same MSLN peptide, or peptides, across two or more biological samples under the same experimental conditions. In addition, relative quantitation can be determined for a given peptide, or peptides, from the MSLN protein within a single sample by comparing the signature peak area for that peptide by SRM/MRM methodology to the signature peak area for another and different peptide, or peptides, from a different protein, or proteins, within the same protein preparation from the biological sample. In this way, the amount of a particular peptide from the MSLN protein, and therefore the amount of the MSLN protein, is determined relative one to another within the same sample.

These approaches permit quantitation of an individual peptide, or peptides, from the MSLN protein to the amount of another peptide, or peptides, between samples and within samples wherein the amounts as determined by signature peak area are relative one to another, regardless of the absolute weight to volume or weight to weight amounts of the MSLN peptide in the protein preparation from the biological sample. Relative quantitative data about individual signature peak areas between different samples are normalized to the amount of protein analyzed per sample. Relative quantitation can be performed across many peptides from multiple proteins and the MSLN protein simultaneously in a single sample and/or across many samples to gain insight into relative protein amounts of one peptide/protein with respect to other peptides/proteins.

Absolute quantitative levels of the MSLN protein are determined by, for example, the SRM/MRM methodology whereby the SRM/MRM signature peak area of an individual peptide from the MSLN protein in one biological sample is compared to the SRM/MRM signature peak area of a spiked internal standard. In one embodiment, the internal standard is a synthetic version of the same exact MSLN peptide that contains one or more amino acid residues labeled with one or more heavy isotopes. Such an isotope labeled internal standard is synthesized so that when analyzed by mass spectrometry it generates a predictable and consistent SRM/MRM signature peak that is different and distinct from the native MSLN peptide signature peak and which can be used as a comparator peak. Thus when the internal standard is spiked into a protein preparation from a biological sample in known amounts and analyzed by mass spectrometry, the SRM/MRM signature peak area of the native peptide is compared to the SRM/MRM signature peak area of the internal standard peptide, and this numerical comparison indicates either the absolute molarity and/or absolute weight of the native peptide present in the original protein preparation from the biological sample. Absolute quantitative data for fragment peptides are displayed according to the amount of protein analyzed per sample. Absolute quantitation can be performed across many peptides, and thus proteins, simultaneously in a single sample and/or across many samples to gain insight into absolute protein amounts in individual biological samples and in entire cohorts of individual samples.

The SRM/MRM assay method can be used to aid diagnosis of the stage of cancer, and/or the patient prognosis, for example, directly in patient-derived tissue, such as formalin fixed tissue, and to aid in determining which therapeutic agent would be most advantageous for use in treating that patient. Cancer tissue that is removed from a patient either through surgery, such as for therapeutic removal of partial or entire tumors, or through biopsy procedures conducted to determine the presence or absence of suspected disease, is analyzed to determine whether or not a specific protein, or proteins, and which forms of proteins, are present in that patient tissue. Moreover, the expression level of a protein, or multiple proteins, can be determined and compared to a “normal” or reference level found in healthy tissue. Normal or reference levels of proteins found in healthy tissue may be derived from, for example, the relevant tissues of one or more individuals that do not have cancer. Alternatively, normal or reference levels may be obtained for individuals with cancer by analysis of relevant tissues not affected by the cancer. Assays of protein levels (e.g., MSLN levels) can also be used to diagnose, and provide prognostic information about, the stage of cancer in a patient or subject diagnosed with cancer by employing the MSLN levels. The level of an individual MSLN peptide is defined as the molar amount of the peptide determined by the SRM/MRM assay per total amount of protein lysate analyzed. Information regarding MSLN can thus be used to aid in determining stage or grade of a cancer by correlating the level of the MSLN protein (or fragment peptides of the MSLN protein) with levels observed in normal tissues. Once the quantitative amount of the MSLN protein has been determined in the cancer cells, that information can be matched to a list of therapeutic agents (chemical and biological) developed to specifically treat cancer tissue that is characterized by, for example, abnormal expression of the protein or protein(s) (e.g., MSLN) that were assayed. Matching information from an MSLN protein assay to a list of therapeutic agents that specifically targets, for example, the MSLN protein or cells/tissue expressing the protein, defines what has been termed a personalized medicine approach to treating disease. The assay methods described herein form the foundation of a personalized medicine approach by using analysis of proteins from the patient's own tissue as a source for diagnostic and treatment decisions.

DETAILED DESCRIPTION

In principle, any predicted peptide derived from the MSLN protein, prepared for example by digesting with a protease of known specificity (e.g. trypsin), can be used as a surrogate reporter to determine the abundance of MSLN protein in a sample using a mass spectrometry-based SRM/MRM assay. Similarly, any predicted peptide sequence containing an amino acid residue at a site that is known to be potentially modified in the MSLN protein also might potentially be used to assay the extent of modification of the MSLN protein in a sample.

MSLN fragment peptides may be generated by a variety of methods including by the use of the Liquid Tissue protocol provided in U.S. Pat. No. 7,473,532. The Liquid Tissue protocol and reagents are capable of producing peptide samples suitable for mass spectroscopic analysis from formalin fixed paraffin embedded tissue by proteolytic digestion of the proteins in the tissue/biological sample. In the Liquid Tissue protocol the tissue/biological is heated in a buffer for an extended period of time (e.g., from about 80° C. to about 100° C. for a period of time from about 10 minutes to about 4 hours) to reverse or release protein cross-linking. The buffer employed is a neutral buffer, (e.g., a Tris-based buffer, or a buffer containing a detergent). Following heat treatment the tissue/biological sample is treated with one or more proteases, including but not limited to trypsin, chymotrypsin, pepsin, and endoproteinase Lys-C for a time sufficient to disrupt the tissue and cellular structure of said biological sample and to liquefy the sample (e.g. a period of time from 30 minutes to 24 hours at a temperature from 37° to 65° C.). The result of the heating and proteolysis is a liquid, soluble, dilutable biomolecule lysate.

Surprisingly, it was found that many potential peptide sequences from the MSLN protein are unsuitable or ineffective for use in mass spectrometry-based SRM/MRM assays for reasons that are not immediately evident. This was especially true for peptides derived from formalin-fixed tissue. As it was not possible to predict the most suitable peptides for MRM/SRM assay, it was necessary to experimentally identify modified and unmodified peptides in actual Liquid Tissue lysates to develop a reliable and accurate SRM/MRM assay for the MSLN protein. While not wishing to be bound by any theory, it is believed that some peptides might, for example, be difficult to detect by mass spectrometry because they either do not ionize well or they produce fragments that are not distinct from peptides derived from other proteins. Peptides may also fail to resolve well in separation (e.g., liquid chromatography), or may adhere to glass or plastic ware.

MSLN peptides found in various embodiments of this disclosure (e.g., Tables 1 and 2) were derived from the MSLN protein by protease digestion of all the proteins within a complex Liquid Tissue lysate prepared from cells procured from formalin fixed cancer tissue. Unless noted otherwise, in each instance the protease was trypsin. The Liquid Tissue lysate was then analyzed by mass spectrometry to determine those peptides derived from the MSLN protein that are detected and analyzed by mass spectrometry. Identification of a specific preferred subset of peptides for mass-spectrometric analysis is based on; 1) experimental determination of which peptide or peptides from a protein ionize in mass spectrometry analyses of Liquid Tissue lysates, and 2) the ability of the peptide to survive the protocol and experimental conditions used in preparing a Liquid Tissue lysate. This latter property extends not only to the amino acid sequence of the peptide but also to the ability of a modified amino acid residue within a peptide to survive in modified form during the sample preparation.

Protein lysates from cells procured directly from formalin (formaldehyde) fixed tissue were prepared using the Liquid Tissue reagents and protocol that entails collecting cells into a sample tube via tissue microdissection followed by heating the cells in the Liquid Tissue buffer for an extended period of time. Once the formalin-induced cross linking has been negatively affected, the tissue/cells are then digested to completion in a predictable manner using a protease, as for example including but not limited to the protease trypsin. Each protein lysate is turned into a collection of peptides by digestion of intact polypeptides with the protease. Each Liquid Tissue lysate was analyzed (e.g., by ion trap mass spectrometry) to perform multiple global proteomic surveys of the peptides where the data was presented as identification of as many peptides as could be identified by mass spectrometry from all cellular proteins present in each protein lysate. An ion trap mass spectrometer or another form of a mass spectrometer that is capable of performing global profiling for identification of as many peptides as possible from a single complex protein/peptide lysate is typically employed. Ion trap mass spectrometers however may be the best type of mass spectrometer for conducting global profiling of peptides. Although an SRM/MRM assay can be developed and performed on any type of mass spectrometer, including a MALDI, ion trap, or triple quadrupole, advantageously the instrument platform for an SRM/MRM assay is a triple quadrupole instrument platform.

Once as many peptides as possible were identified in a single MS analysis of a single lysate under the conditions employed, then that list of peptides was collated and used to determine the proteins that were detected in that lysate. That process was repeated for multiple Liquid Tissue lysates, and the very large list of peptides was collated into a single dataset. That type of dataset can be considered to represent the peptides that can be detected in the type of biological sample that was analyzed (after protease digestion), and specifically in a Liquid Tissue lysate of the biological sample, and thus includes the peptides for specific proteins, such as for example the MSLN protein.

In one embodiment, the MSLN tryptic peptides identified as useful in the determination of absolute or relative amounts of the MSLN protein include one or more of SEQ ID NO:1 and SEQ ID NO:2, both of which are listed in Table 1. Each of these peptides was detected by mass spectrometry in Liquid Tissue lysates prepared from formalin fixed, paraffin embedded tissue. Thus, each peptide is a candidate for use in developing a quantitative SRM/MRM assay for the MSLN protein in human biological samples, including directly in formalin fixed patient tissue.

TABLE 1 Mono Precursor Isotopic Charge Precursor Transition Inn SEQ ID Peptide sequence Mass State m/z m/z Type SEQ ID NO: 1 VNAIPFTYEQLDVLK 1748.9348 2 875.475 676.858 y11 2 875.475 844.477 y7 2 875.475 1007.54 y8 2 875.475 1108.588 y9 2 875.475 1352.709 y11 SEQ ID NO: 2 GSLLSEADVR 1045.5403 2 523.777 514.772 b9 2 523.777 589.294 y5 2 523.777 676.326 y6 2 523.777 789.41 y7 The MSLN tryptic peptides listed in Table 1 were detected from multiple Liquid Tissue lysates of multiple different formalin fixed tissues of different human organs including prostate, colon, and breast. Each of those peptides is considered useful for quantitative SRM/MRM assay of the MSLN protein in formalin fixed tissue. Further data analysis of these experiments indicated no preference is observed for either of these peptides from any specific organ site. Thus, each of these peptides is believed to be suitable for conducting SRM/MRM assays of the MSLN protein on a Liquid Tissue lysate from any formalin fixed tissue originating from any biological sample or from any organ site in the body.

An important consideration when conducting an SRM/MRM assay is the type of instrument that may be employed in the analysis of the peptides. Although SRM/MRM assays can be developed and performed on any type of mass spectrometer, including a MALDI, ion trap, or triple quadrupole, the most advantageous instrument platform for an SRM/MRM assay is presently most likely considered to be a triple quadrupole instrument platform. That type of a mass spectrometer is presently the most suitable instrument for putative analysis of a single isolated target peptide within a very complex protein lysate that may consist of hundreds of thousands to millions of individual peptides from all the proteins contained within a cell.

In order to most efficiently implement an SRM/MRM assay for each peptide derived from the MSLN protein it is desirable to utilize information in addition to the peptide sequence in the analysis. That additional information may be used in directing and instructing the mass spectrometer (e.g. a triple quadrupole mass spectrometer) to perform the correct and focused analysis of specific targeted peptide(s), such that the assay may be effectively performed.

The additional information about target peptides in general, and about specific MSLN peptides, may include one or more of the mono isotopic mass of the peptide, its precursor charge state, the precursor m/z value, the m/z transition ions, and the ion type of each transition ion. Additional peptide information that may be used to develop an SRM/MRM assay for the MSLN protein is shown for these MSLN peptides in Table 1.

The method described below was used to: 1) identify candidate peptides from the MSLN protein that can be used for a mass spectrometry-based SRM/MRM assay for the MSLN protein, 2) develop an individual SRM/MRM assay, or assays, for target peptides from the MSLN protein in order to correlate and 3) apply quantitative assays to cancer diagnosis and/or choice of optimal therapy.

Assay Method

-   1. Identification of SRM/MRM candidate fragment peptides for the     MSLN protein     -   a. Prepare a Liquid Tissue protein lysate from a formalin fixed         biological sample using a protease or proteases, (that may or         may not include trypsin), to digest proteins     -   b. Analyze all protein fragments in the Liquid Tissue lysate on         an ion trap tandem mass spectrometer and identify all fragment         peptides from the MSLN protein, where individual fragment         peptides do not contain any peptide modifications such as         phosphorylations or glycosylations     -   c. Analyze all protein fragments in the Liquid Tissue lysate on         an ion trap tandem mass spectrometer and identify all fragment         peptides from the MSLN protein that carry peptide modifications         such as for example phosphorylated or glycosylated residues     -   d. All peptides generated by a specific digestion method from         the entire, full length MSLN protein potentially can be         measured, but preferred peptides used for development of the         SRM/MRM assay are those that are identified by mass spectrometry         directly in a complex Liquid Tissue protein lysate prepared from         a formalin fixed biological sample     -   e. Peptides that are specifically modified (phosphorylated,         glycosylated, etc.) in patient tissue and which ionize, and thus         detected, in a mass spectrometer when analyzing a Liquid Tissue         lysate from a formalin fixed biological sample are identified as         candidate peptides for assaying peptide modifications of the         MSLN protein -   2. Mass Spectrometry Assay for Fragment Peptides from the MSLN     Protein     -   a. SRM/MRM assay on a triple quadrupole mass spectrometer for         individual fragment peptides identified in a Liquid Tissue         lysate is applied to peptides from the MSLN protein         -   i. Determine optimal retention time for a fragment peptide             for optimal chromatography conditions including but not             limited to gel electrophoresis, liquid chromatography,             capillary electrophoresis, nano-reversed phase liquid             chromatography, high performance liquid chromatography, or             reverse phase high performance liquid chromatography         -   ii. Determine the mono isotopic mass of the peptide, the             precursor charge state for each peptide, the precursor m/z             value for each peptide, the m/z transition ions for each             peptide, and the ion type of each transition ion for each             fragment peptide in order to develop an SRM/MRM assay for             each peptide.         -   iii. SRM/MRM assay can then be conducted using the             information from (i) and (ii) on a triple quadrupole mass             spectrometer where each peptide has a characteristic and             unique SRM/MRM signature peak that precisely defines the             unique SRM/MRM assay as performed on a triple quadrupole             mass spectrometer     -   b. Perform SRM/MRM analysis so that the amount of the fragment         peptide of the MSLN protein that is detected, as a function of         the unique SRM/MRM signature peak area from an SRM/MRM mass         spectrometry analysis, can indicate both the relative and         absolute amount of the protein in a particular protein lysate.         -   i. Relative quantitation may be achieved by:             -   1. Determining increased or decreased presence of the                 MSLN protein by comparing the SRM/MRM signature peak                 area from a given MSLN peptide detected in a Liquid                 Tissue lysate from one formalin fixed biological sample                 to the same SRM/MRM signature peak area of the same MSLN                 fragment peptide in at least a second, third, fourth or                 more Liquid Tissue lysates from least a second, third,                 fourth or more formalin fixed biological samples             -   2. Determining increased or decreased presence of the                 MSLN protein by comparing the SRM/MRM signature peak                 area from a given MSLN peptide detected in a Liquid                 Tissue lysate from one formalin fixed biological sample                 to SRM/MRM signature peak areas developed from fragment                 peptides from other proteins, in other samples derived                 from different and separate biological sources, where                 the SRM/MRM signature peak area comparison between the 2                 samples for a peptide fragment are normalized to amount                 of protein analyzed in each sample.             -   3. Determining increased or decreased presence of the                 MSLN protein by comparing the SRM/MRM signature peak                 area for a given MSLN peptide to the SRM/MRM signature                 peak areas from other fragment peptides derived from                 different proteins within the same Liquid Tissue lysate                 from the formalin fixed biological sample in order to                 normalize changing levels of MSLN protein to levels of                 other proteins that do not change their levels of                 expression under various cellular conditions.             -   4. These assays can be applied to both unmodified                 fragment peptides and for modified fragment peptides of                 the MSLN protein, where the modifications include but                 are not limited to phosphorylation and/or glycosylation,                 and where the relative levels of modified peptides are                 determined in the same manner as determining relative                 amounts of unmodified peptides.         -   ii. Absolute quantitation of a given peptide may be achieved             by comparing the SRM/MRM signature peak area for a given             fragment peptide from the MSLN protein in an individual             biological sample to the SRM/MRM signature peak area of an             internal fragment peptide standard spiked into the protein             lysate from the biological sample             -   1. The internal standard is a labeled synthetic version                 of the fragment peptide from the MSLN protein that is                 being interrogated. This standard is spiked into a                 sample in known amounts, and the SRM/MRM signature peak                 area can be determined for both the internal fragment                 peptide standard and the native fragment peptide in the                 biological sample separately, followed by comparison of                 both peak areas             -   2. This can be applied to unmodified fragment peptides                 and modified fragment peptides, where the modifications                 include but are not limited to phosphorylation and/or                 glycosylation, and where the absolute levels of modified                 peptides can be determined in the same manner as                 determining absolute levels of unmodified peptides. -   3. Apply Fragment Peptide Quantitation to Cancer Diagnosis and     Treatment     -   a. Perform relative and/or absolute quantitation of fragment         peptide levels of the MSLN protein and demonstrate that the         previously-determined association, as well understood in the         field of cancer, of MSLN protein expression to the         stage/grade/status of cancer in patient tumor tissue is         confirmed     -   b. Perform relative and/or absolute quantitation of fragment         peptide levels of the MSLN protein and demonstrate correlation         with clinical outcomes from different treatment strategies,         wherein this correlation has already been demonstrated in the         field or can be demonstrated in the future through correlation         studies across cohorts of patients and tissue from those         patients. Once either previously established correlations or         correlations derived in the future are confirmed by this assay         then the assay method can be used to determine optimal treatment         strategy

Specific and unique characteristics about specific MSLN peptides were developed by analysis of all MSLN peptides on both an ion trap and triple quadrupole mass spectrometers. That information includes the monoisotopic mass of the peptide, its precursor charge state, the precursor m/z value, the transition m/z values of the precursor, and the ion types of each of the identified transitions. That information must be determined experimentally for each and every candidate SRM/MRM peptide directly in Liquid Tissue lysates from formalin fixed samples/tissue; because, interestingly, not all peptides from the MSLN protein can be detected in such lysates using SRM/MRM as described herein, indicating that MSLN peptides not detected cannot be considered candidate peptides for developing an SRM/MRM assay for use in quantitating peptides/proteins directly in Liquid Tissue lysates from formalin fixed samples/tissue.

A particular SRM/MRM assay for a specific MSLN peptide is performed on a triple quadrupole mass spectrometer. An experimental sample analyzed by a particular MSLN SRM/MRM assay is for example a Liquid Tissue protein lysate prepared from a tissue that had been formalin fixed and paraffin embedded. Data from such as assay indicates the presence of the unique SRM/MRM signature peak for this MSLN peptide in the formalin fixed sample.

Specific transition ion characteristics for this peptide are used to quantitatively measure a particular MSLN peptide in formalin fixed biological samples. These data indicate absolute amounts of this MSLN peptide as a function of molar amount of the peptide per microgram of protein lysate analyzed. Assessment of MSLN protein levels in tissues based on analysis of formalin fixed patient-derived tissue can provide diagnostic, prognostic, and therapeutically-relevant information about each particular patient. In one embodiment, this disclosure describes a method for measuring the level of the Mesothelin protein (MSLN) in a biological sample, comprising detecting and/or quantifying the amount of one or more modified or unmodified MSLN fragment peptides in a protein digest prepared from the biological sample using mass spectrometry; and calculating the level of modified or unmodified MSLN protein in the sample; and wherein the level is a relative level or an absolute level. In a related embodiment, quantifying one or more MSLN fragment peptides comprises determining the amount of the each of the MSLN fragment peptides in a biological sample by comparison to an added internal standard peptide of known amount, wherein each of the MSLN fragment peptides in the biological sample is compared to an internal standard peptide having the same amino acid sequence. In some embodiments the internal standard is an isotopically labeled internal standard peptide comprises one or more heavy stable isotopes selected from ¹⁸O, ¹⁷O, ³⁴S, ¹⁵N, ¹³C, ²H or combinations thereof.

The method for measuring the level of the MSLN protein in a biological sample described herein (or fragment peptides as surrogates thereof) may be used as a diagnostic and/or prognostic indicator of cancer in a patient or subject. In one embodiment, the results from measurements of the level of the MSLN protein may be employed to determine the diagnostic stage/grade/status of a cancer by correlating (e.g., comparing) the level of MSLN protein found in a tissue with the level of that protein found in normal and/or cancerous or precancerous tissues.

Because both nucleic acids and protein can be analyzed from the same Liquid Tissue™ biomolecular preparation it is possible to generate additional information about disease diagnosis and drug treatment decisions from the nucleic acids in same sample upon which proteins were analyzed. For example, if the MSLN protein is expressed by certain cells at increased levels, when assayed by SRM the data can provide information about the state of the cells and their potential for uncontrolled growth, potential drug resistance and the development of cancers can be obtained. At the same time, information about the status of the MSLN genes and/or the nucleic acids and proteins they encode (e.g., mRNA molecules and their expression levels or splice variations) can be obtained from nucleic acids present in the same Liquid Tissue™ biomolecular preparation can be assessed simultaneously to the SRM analysis of the MSLN protein. Any gene and/or nucleic acid not from the MSLN and which is present in the same biomolecular preparation can be assessed simultaneously to the SRM analysis of the MSLN protein. In one embodiment, information about the MSLN protein and/or one, two, three, four or more additional proteins may be assessed by examining the nucleic acids encoding those proteins. Those nucleic acids can be examined, for example, by one or more, two or more, or three or more of: sequencing methods, polymerase chain reaction methods, restriction fragment polymorphism analysis, identification of deletions, insertions, and/or determinations of the presence of mutations, including but not limited to, single base pair polymorphisms, transitions, transversions, or combinations thereof. 

The invention claimed is:
 1. A method for measuring the level of the Mesothelin protein (MSLN) in a human biological sample of formalin-fixed tissue, comprising detecting and quantifying the amount of a MSLN fragment peptide in a protein digest prepared from said biological sample, using mass spectrometry; wherein said MSLN fragment peptide is a peptide having the sequence set forth in SEQ ID NO:1 or SEQ ID NO:2, and calculating the level of MSLN protein in said sample; and wherein said level is a relative level or an absolute level.
 2. The method of claim 1, further comprising the step of fractionating said protein digest prior to detecting and quantifying the amount of MSLN fragment.
 3. The method of claim 2, wherein said fractionating step is selected from the group consisting of liquid chromatography, nano-reversed phase liquid chromatography, high performance liquid chromatography, or reverse phase high performance liquid chromatography.
 4. The method of claim 1, wherein said protein digest comprises a protease digest.
 5. The method of claim 4, wherein said protein digest comprises a trypsin digest.
 6. The method of claim 1, wherein the tissue is paraffin embedded tissue.
 7. The method of claim 1, wherein the tissue is obtained from a tumor.
 8. The method of claim 1, wherein quantifying said MSLN fragment peptide comprises comparing an amount of the peptide of SEQ ID NO:1 or SEQ ID NO:2 in one biological sample to the amount of the same MSLN fragment peptide in a different and separate biological sample.
 9. The method of claim 8, wherein quantifying said MSLN fragment peptide comprises determining the amount of said MSLN fragment peptide in a biological sample by comparison to an added internal standard peptide of known amount, wherein each MSLN fragment peptides in the biological sample is compared to an internal standard peptide having the same amino acid sequence.
 10. The method of claim 9, wherein the internal standard peptide is an isotopically labeled peptide.
 11. The method of claim 10, wherein the isotopically labeled internal standard peptide comprises one or more heavy stable isotopes selected from the group consisting of ¹⁸O, ¹⁷O, ³⁴S, ¹⁵N, ¹³C, ²H and combinations thereof.
 12. The method of claim 1, further comprising selecting for the subject from which said biological sample was obtained a treatment based on the presence, absence, or amount of one or more MSLN fragment peptide or the level of MSLN protein.
 13. The method according to claim 12, further comprising administering to the patient from which said biological sample was obtained a therapeutically effective amount of a therapeutic agent, wherein the therapeutic agent and/or amount of the therapeutic agent administered is based upon amount of MSLN fragment peptide or the level of MSLN protein.
 14. The method of claim 13, wherein therapeutic agents bind the MSLN protein and/or inhibit its biological activity. 